Organic Chemistry

Section 2.10 Conformations of Alkanes: Rotation About Carbon–Carbon Bonds 89

will use Newman projections because they are easy to draw and they do a good job of

representing the spatial relationships of the substituents on the two carbon atoms.

The electrons in a bond will repel the electrons in another bond if the

bonds get too close to each other. The staggered conformation, therefore, is the most

stable conformation of ethane because the bonds are as far away from each

other as possible. The eclipsed conformationis the least stable conformation because

in no other conformation are the bonds as close to one another. The extra ener-

gy of the eclipsed conformation is called torsional strain. Torsional strainis the name

given to the repulsion felt by the bonding electrons of one substituent as they pass close

to the bonding electrons of another substituent. The investigation of the various confor-

mations of a compound and their relative stabilities is called conformational analysis.

Rotation about a carbon–carbon single bond is not completely free because of the

energy difference between the staggered and eclipsed conformers. The eclipsed con-

former is higher in energy, so an energy barrier must be overcome when rotation about

the carbon–carbon bond occurs (Figure 2.4). However, the barrier in ethane is small

C¬H

C¬H

C¬H C¬H

Melvin S. Newman (1908–1993)

was born in New York. He received

a Ph.D. from Yale University in

1932 and was a professor of

chemistry at Ohio State University

from 1936 to 1973.

HH

H^60 °

60 °

60 °

H

HH

H

H

H

H

H

H

H

H

H

H

H

H

CC

HH

H H

HH

CC

H

H H

H

H

H

H

H H

H

HH

perspective

formulas

sawhorse

projections

Newman

projections

staggered conformation for

rotation about the carbon–carbon

bond in ethane

eclipsed conformation for

rotation about the carbon–carbon

bond in ethane

eclipsed conformers

0 ° 60 ° 120 °

staggered conformers

eclipsed conformers

180 °

Degrees of rotation

240 ° 300 ° 360 °

Potential energy

2.9 kcal/mol

or 12 kJ/mol

H

H

H

HH

H

H

HH

H H

H

H

H H

H

H H

H

H

H

H

H H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

Figure 2.4

Potential energy of ethane as a function of the angle of rotation about the carbon–carbon bond.

3-D Molecule:

Staggered and eclipsed

conformations of ethane

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